LONDON – Tropical ecosystems may be responding to global warming more energetically than anyone had expected.

Scientists from China, Germany, France, the United Kingdom and the United States report in the journal Nature that the tropical carbon cycle – the uptake and release of carbon dioxide from and back into the atmosphere – has become twice as sensitive to temperature change in the last 50 years.

A one degree rise in average tropical temperature leads to a release of around two billion more tons of carbon per year from tropical forests and savannahs, compared with the 1960s and 1970s.

This is unexpected. Climate scientists had foreseen the ability of land-based ecosystems to store carbon declining through the coming century as average global temperatures rise, but not on this scale.

Positive feedback loop
To arrive at their conclusions, the researchers looked not at the crude rise in carbon dioxide levels in the atmosphere, but the year-to-year variations of traces of the gas recorded both at the top of the Mauna Loa volcano in Hawaii and at the South Pole, and then matched these with mean annual temperature variations over the same timescale.

Together, the findings seem to show that tropical ecosystems are becoming more sensitive to climate change. This could be another example of what engineers call positive feedback – warmer summers make forests drier and release more carbon dioxide to make summers warmer, and so on.

There is evidence that the tropical regions have experienced more drought over the last five decades. More to the point, the team’s conclusion really suggests that if existing climate models provide uncertain projections of the future, it is because the information available is still incomplete.

It does however also indicate that variations in carbon dioxide levels would provide a monitor of the way tropical forests and grasslands are responding to climate.

"This enhancement is very unlikely to have resulted from chance, and may provide a new perspective on a possible shift in the terrestrial carbon cycle over the past five decades," said Xuhui Wang of Peking University in Beijing, who led the study.

Added Pierre Friedlingstein, of the University of Exeter in the United Kingdom and a member of the research team: "Current land carbon cycle systems do not show this increase over the last 50 years, because these models underestimate emerging drought effects on tropical ecosystems."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.